Bonded Phases - Application of the Theory to the Adsorption of Aliphatic Acids

 

Application of the Theory to the Adsorption of Aliphatic Acids

 

The data obtained for acetic acid and propionic acid were processed in a similar way and the curves relating the reciprocal of the retention volume of each acid to the concentration of the acid are shown in figure 18.

Figure 18. Graph of the 1/V' for a Acetic and Propionic Acids against their Respective Concentration in the Mobile Phase

It is seen that the same linear relationship is realized between the reciprocal of the corrected retention volume and the solvent concentration of in the mobile phase. Using the values for the slopes and intercepts obtained from the curve fitting procedure as before. And employing the same equations, the effective surface area of the reverse phase that was available to both acids was calculated and in addition the distribution coefficient of each acid between water and the reverse phase. The results are given in table 5. The results indicate that the surface area available to the acids is significantly less than that available to the alcohols. However, this could be because the aliphatic acids are partially ionized which would cause them to be ionically excluded from the pores of the stationary phase. Under such circumstances only the surface chains of the reverse phase would be available for interaction. This would explain why propionic acid, (the weaker acid and less ionized) is the least excluded and appears to have access to significantly more reverse phase than acetic acid.

 

Table 5. Chromatographic Data for the Two Aliphatic Acids

 

 

Using the data given in table 5 in conjunction with equation used for the alcohols the adsorption isotherm equations were calculated for both acids.

 

. .................. (16)

 

Propionic Acid ................. (17)

 

In a manner similar to that employed with the alcohols, equations (16) and (17) were used to construct the adsorption isotherms for the two aliphatic acids that are shown in figure 19. It is seen that the acids exhibit the same type of isotherm as methanol and ethanol, the first two members of the alcohol series. It is also apparent that when the solvent mixture contains about 10 %w/v of acid the reverse phase will be covered with carboxyl groups from the adsorbed acid. It must be clearly understood, however, that the term solvent and solute, in the sense discussed here, are virtually synonymous.